As known, rotary positive displacement pumps are used, above all, in hydraulic field, in order to transfer energy to a fluid designed to operate a facility.
Such pumps comprise a casing provided with a suction port and a discharge port, at least a pair of shafts having rotary meshing toothings being housed inside the casing. A plurality of chambers are defined among the meshing teeth by virtue of the rotation, the volume of the chambers varying in the meshing zone so that the fluid is caused to be transferred from the suction side to the delivery side.
The toothed wheels of the positive displacement pumps are usually comprised of straight tooth spur gears that are not expensive.
Thanks to their simple construction, such toothed wheels are very cheap indeed. However, such pumps are subject to drawbacks as they do not deliver the fluid in a constant way due to the straight tooth spur gears, and further they make much noise. These drawbacks depend on that the meshing engages the teeth with a discontinuity typical of a discrete variation (for example, there are one to two meshing teeth when the transverse contact ratio εα<2, and two to three meshing teeth for 2<εα<3). This discontinuity causes both mechanical and hydraulic noise. The mechanical noise results from the discontinuity of the meshing mode, and the hydraulic noise depends mainly on that the fluid is transferred in a non-constant way due to distinctive ripples that also cause vibrations in the plant served by the pump. Further, the pumps having both straight tooth gears and involute (but also cycloid) standard helical tooth gears have a problem of a closed space between the tooth bottom land of a toothing, and the tooth top land of a conjugate toothing. This closed space changes during the meshing so that sharp pressure variations in the fluid are provoked. Such a drawback is reduced by means of suitable escape passageways made on side shims or support faces.
Furthermore a reduction of the drawbacks is obtained by means of more valuable helical toothing, in which the contact occurs gradually and with gradually varying lengths along skew lines with respect to the rotation axes. The overlapping of the different contact lines during the meshing makes these toothing very soft in their operation so that an irregular delivery is lessened. The problem of hydraulic irregularity and that one of the trapped fluid are completely overcome by means of so called “continuous contact” special helical profiles having rounded tooth top and bottom. Such profiles by virtue of their specific shape characterised, among other, by the absence of sharp edges, do not encapsulate fluid between the tooth top and the bottom of the conjugate tooth so that the trapped fluid problem is eliminated and the discontinuity of fluid delivery is almost annulled thanks to a suitable choice of the helical contact ratio.
Such profiles are made functional and industrially suitable in applications for high pressures according to teachings of the patents EP1132618, EP1371848 and BO2009A000714 of the present inventor; the last one of these patents is a development of the two preceding patents, and defines so called semi-incapsulating profiles. However, the implementation of these profiles does not solve the problem caused by axial forces resulting from the helical toothings, problem that is overcome by adopting those profiles but in the scope of the present invention, since in the known pumps the use of helical profiles causes axial forces of both mechanical and hydraulic nature. These axial forces, as they can not be completely adjusted, cause an inevitable worsening of the side faces of the toothings and of the support bushings. Further mechanical losses by friction occur with a consequence of reduction of the mechanical efficiency. These drawbacks can be overcome by adopting even more precious double-helical toothings. The double-helical profile allows the axial force resulting from the use of the single helical profile to be balanced, as the two helical profiles are identical and a mirror image of each other with respect to a center line plane of the toothing perpendicular to the axes of rotation.
Also these pumps are not free from drawbacks such as the production cost which is very onerous due to the high level of accuracy requested. Further, this accuracy can be achieved only by means of sophisticated machine tools, as, for example, the gear-cutting machine Sykes that uses a fly cutter but usually does not allow hardened material to be machined. As known, double-helical gears can be obtained through traditional gear-cutting machines and then ground by a technology adapted to high superficial hardness materials. Such gears have a double-helical toothing divided by a toothing free undercutting channel that is generally symmetrical to the center line plane of the profiles and causes heavy inefficiency in liquid sealing. In double-helical toothing it should be suitable from the economical and technological points of view to use simple helical wheels having side by side assembled specular helicals. A main drawback of such a solution consists of the high accuracy requested in relative and absolute positioning of the helical wheel, as each wheel must be in phase with the flanked one and both wheels must be in phase with the conjugate wheels. Also specularity planes of the toothings must be coincident. This implies a first restraint defined by the need of putting in phase the adjacent driving toothing, a second restraint defined by the need of putting in phase the adjacent driven toothing, a third restraint defined by the need of coincidence of the specularity planes, and a fourth restraint represented by the coplanarity of the side faces of the wheels, since they must seal on the side planes of the support bushings or the housing. From said drawbacks it results that the double-helical pumps are difficult to be made and unsatisfactory in their performance: at the same level of accuracy they are less performing at high pressures and generally noisier than the others.
In short and schematically, the drawbacks of the known gear positive displacement pumps are at least the following ones:    A—mechanical noise    B—hydraulic noise and vibrations caused by ripples    C—hydraulic noise and vibrations caused by variations in pressure of the trapped fluid    D—axial forces that can not be completely balanced in the helical pumps    E—low efficiency of continuous contact helical profile pumps    F—too many restraints and construction problems of the double-helical pumps.
In particular, the straight tooth gear pumps have the drawbacks as to preceding items A, B, and C.
The involute helical toothing pumps solve the problem as to item A, they reduce the problem as to item B, they worsen the problem as to item C and further have the problem as to item D.
The continuous contact helical profile pumps solve the problem as to item A, they solve the problem as to item B, they solve the problem as to item C, they solve the problem as to item D but they do not solve the problem as to item E, so that they can not be used for high pressures.
The helical toothing pumps with profiles such as the ones described in the already cited patents EP1132618, EP1371848, and BO2009A000714 solve the problem as to item A, they solve the problem as to item B, they solve the problem as to item C, they have the problem as to item D and they solve the problem as to item E.
The involute double-helical pumps solve theoretically but often not practically the problem as to item A, as, if they are not manufactured and assembled with extreme accuracy, they mesh incorrectly, they reduce the problem as to item B, they do not solve the problem as to item C, they solve the problem as to item D, they do not have the problem as to item E, but they suffer the problem as to item F.